X, Y and Z axes
It has been reported in the literature [22] that the use of
aerostatic slides are the best way to reach precise and accurate
motion in micro manufacturing applications. Air bearings
feature frictionless and backlash-free motion, high acceleration
capabilities, high accuracy and easy maintenance. This
system implements a direct drive system, which has proven to
be the best choice in precision machining applications [32].
Custom aerostatic slides were designed manufactured to
achieve the best performance (Fig. 3). Each of the slides
consists of a DC brushless linear motor with a trapezoidal hall
sensor, a linear encoder readhead with its scale and an air
bearing. The air bearings consist of a sliding cage which is
made of brass assembled around a granite beam. A very
specific groove pattern has been machined on the air bearing
surface to improve stability [33, 34]. The clearance between
the brass plates and the beam surface has been optimized for
better performance of the slides. These slides are actuated by
linear brushless DC motors model LEM-S-3-S from Anorad.
A trapezoidal hall effect sensor is used to find the phase at
motor start up [35]. The motors therefore have two feedback
sensors for control: (1) the trapezoidal Hall Effect sensor and
(2) the linear position encoder.
Both of these feedback devices are interfaced with the
motion control system via an interface board Delta Tau
ACC24E-2, which can control up to four axes simultaneously.
Linear motors have several advantages over screw driven
systems:
& High velocity
& High acceleration
& Smoothness of motion (no friction)
& High accuracy and repeatability
& High stiffness
& Low maintenance and high life expectancy
& Possibility for clean room and vacuum applications
For the Z-axis, the linear motor is mounted vertically, and
the weight of the slide is supported by frictionless air cylinders.
The weight of the Z slide is cancelled by adjusting the
pressure into the air cylinders so that the force of the motor is
only used to move the slide. This avoids motor overheating
and prevents the tool from crashing into the workpiece in case
of power loss. Brakes are also mounted onto the air cylinders
and are activated if there is no air pressure, which prevents the
slide from crashing.
The encoder receiver frequency of the motion control
system is set to 40 MHz, which allows a maximum speed of
0.054 m/s on each axis. This speed is more than sufficient
because the tool feed rates during the machining process is
very low. The 2-nm encoder resolution of the position feedback
system allows a very precise motion of the slides. A
reference mark is located in the middle of the scale and is used
at initialization to setup the axis (during phase and home
search).The encoder system is made of a read head, an interpolator
and a scale. The selected models of these components are
summarized in Table 1.
The slides are actuated using direct PWM amplifiers
Geobrick from Delta Tau. Each drive can handle two axes
and is directly controlled by the PWMcontrol signals coming
from the Power PMAC motion controller. The PWMfrequency
has been set to 12 kHz, which is the recommended frequency
for DC brushless linear motors.